Fossil sirenians are well known from the Western Atlantic and Caribbean (WAC) region. Neogene records from the Atlantic coast of South America, although scarce, seem to reflect a similar taxonomic composition to its northern contemporaries. Fossil sirenians from Argentina are known from the late Miocene Paraná and Ituzaingó formations in Entre Ríos Province. An upper third molar housed in the Museo de Paleontología de la Universidad Nacional de Córdoba (CORD-PZ 4301), from the Paraná Formation, was originally described and assigned to the dugongid genus Metaxytherium Christol. Subsequent workers suggested it could instead belong to the dugongid genus Dioplotherium Cope, a designation that we confirm upon examination of the material. Additionally, we describe new sirenian remains from the Paraná Formation, consisting of two incomplete maxillae with teeth, belonging to one individual, deposited at the Museo de Ciencias Naturales de Paraná (MASP 373). This specimen shows similarities with species of Metaxytherium from the northern WAC. The Paraná Formation sirenians represent the southernmost occurrences of these two genera, and the geologically youngest occurrence of dugongids in the southern WAC. Dioplotherium and Metaxytherium also occur in the early Miocene of northern Brazil. The presence of these taxa as far south as Entre Ríos latitude suggests two possible dispersal routes: (1) across a marginal connection between a southern arm of the Amazonian Sea and the “Paranense Sea”; (2) along the Brazilian coast. Lastly, the occurrence of dugongids, which feed mainly on seagrasses, indicates that these marine angiosperms were present in the region as well.
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